This invention relates generally to a wavelength detector and a polarization sensor that can be used to measure the wavelength or the polarization state of a monochromatic optical signal. Specifically, it provides a means to adjust the properties of a beamsplitter so that it is better suited to either one or the other task. This invention can be used in the fields of optics, telecommunications, and laser spectroscopy.
Wavelength measurement devices that are used to detect, monitor and control a laser""s wavelength and polarization are emerging as integral components of laser optical systems. There are growing demands for wavelength and polarization measurement devices because the telecommunications, spectroscopic, and analytical chemistry industries have grown to the point where accurate management of these properties is essential. The development of dense wavelength division multiplexing (DWDM) systems in telecommunications, and high sensitivity spectroscopic systems in analytical chemistry has led to a demand for wavelength and polarization measurement and control systems that are fast, accurate, give real time wavelength readouts, and are inexpensive.
In optical telecommunications systems, for example, the laser light sources have to be held to a wavelength that moves by less than 1 GHz and at a constant polarization if they are to operate in DWDM systems that have wavelength spacings of 100 GHz or less. This process is achieved using wavelength and polarization measurement devices that samples the output of a laser and provide a signal that can be used to adjust the laser""s properties to the correct value and limit its deviation from that value. Additionally, wavelength measurement devices are used to accurately switch the laser wavelength from one telecommunications channel (ITU channel) to another while maintaining the polarization state. Rapid and accurate wavelength switching (microseconds to nanosecond switching times) over a wide wavelength range (40 nm at 1550 nm) is emerging as a new, and essential requirement for DWDM systems.
In spectroscopy and analytical chemistry, concentrations of chemical constituents or molecular and atomic components can be easily and inexpensively measured with tunable diode lasers provided there exists an accurate and reasonably rapid means to tune the laser to the required wavelength.
Furthermore, the polarization of the laser light on entrance and exit of an optical system needs to be monitored and controlled so that its trajectory through the system can be optimized.
Historically, wavelength measurements have been performed in several ways.
1. A prism or diffraction grating is used to disperse different wavelengths into different directions, each direction corresponding to a unique wavelength. By scanning the directions with a slit and a detector sensitive to light intensity, the wavelength properties of an optical signal can be determined.
2. Alternatively, a scanning optical interferometer can be employed, typically a Michelson interferometer. The wavelength of an optical signal is determined by changing the length of the interferometer by a known amount, and counting the number of interference fringes (a narrow bandwidth optical signal is assumed).
To determine the wavelength of an optical signal with very high accuracy, these measurement techniques must be augmented by a calibration measurement where:
a) the unknown wavelength is compared to a spectroscopic signal; or
b) the unknown wavelength is compared to stabilized laser signal.
Both measurement methods required mechanical movement. Therefore, considerable time is required for the measurement to be performed, typically on the order of seconds. Also, due to requirements for stable mechanical accuracies at micron dimensions, expensive and bulky mechanical components are required. With accurate calibration, great accuracies can be achieved with these techniques, better than 1*10xe2x88x9213 meter resolution, however the time to take the measurements limited their usage in many applications. The need for mechanical stability and repeatability, combined with the complexity of calibration measurements, ensures that the measurement devices are bulky and costly, restricting their usage.
Methods have been developed that partially solve some of these problems. If measurements are to be performed on a monochromatic signal (i.e., one where the bandwidth is very small compared to the center frequency) at a specific wavelength, a dielectric bandpass filter or a Fabry-Perot etalon can be used in place of a wavelength reference. When an optical signal is incident on a dielectric filter or a Fabry-Perot etalon at or near a resonance, transmission through (or reflection from) the filter or etalon is determined by the wavelength of the optical signal and the resonant characteristics of the dielectric filter or etalon. This is illustrated in FIGS. 1 and 2 which show a dielectric filter 11a, FIG. 1, and an etalon 11b, FIG. 2, in a wavelength measurement system. The light beam is applied to a beamsplitter 12. Light from the beamsplitter is applied directly to photodiode 13 and to a second photodiode 14 after it has been transmitted (or reflected) by the dielectric filter 11a or etalon 11b. The outputs of the photodiodes are then compared. The use of a beamsplitter and photodiodes eliminates any error due to changes in beam intensity. A measurement of the transmission (or reflection) therefore determines the wavelength of the light, except that the same transmission (or reflection) can correspond to different wavelengths. When the wavelength of the source is approximately known, as is sometimes the case, this is not a problem. More generally, it means that the wavelength has been determined to be one of several values. For the case of a dielectric bandpass filter with a single transmission peak, the wavelength is determined to be one of two values. In the case of a Fabry-Perot etalon with multiple resonances, the wavelength can be one of multiple values.
These methods make possible wavelength locking of a monochromatic laser light source to a desired wavelength. When the wavelength of the source differs from the desired wavelength, that selected by the filter or etalon, the transmission of the light through the filter or etalon differs from the desired transmission as determined by the output of the two photodiodes. The difference is used to provide an error correction signal that can be used to adjust the wavelength of the laser source to the correct value.
If a bandpass dielectric filter is used for wavelength measurement or wavelength locking, the disadvantage is that it works over a limited wavelength range (typically  less than 2 nm). This means that it cannot be used for applications such a tunable telecommunications lasers where wide wavelength tunability is an important requirement. It also has the disadvantage that the wavelength resolution is dependent on the quality of the thin film coating, which often has ripple or etalon effects that limit the accuracy with which the transmission can be measured.
If a Fabry-Perot etalon is used for wavelength measurement or wavelength locking, it allows wavelength measurement and wavelength locking at a number of different wavelengths corresponding to different resonances of the Fabry-Perot. A disadvantage is that measurement accuracies are restricted by mechanical and thermal stability. Another disadvantage is that Fabry-Perot etalon have multiple resonant transmission peaks and therefore multiple wavelengths that give the same transmission. Thus, the absolute wavelength of the measurement cannot be determined. An additional disadvantage is that the measurement accuracy obtainable with an etalon is very poor at wavelengths situated halfway between its resonances. For telecommunications systems, Fabry-Perot etalon dimensions are inversely proportional to channel spacings.
Polarization sensors have been either devices that preferentially direct one polarization of light along one trajectory and the other polarization along a second path, or devices that only allow one polarization to pass and destroy the other. Both of these devices are susceptible to laser intensity noise, as they both determine the best orientation of the laser polarization with respect to the device by measuring the intensity of the field that is transmitted.
Objects and Advantages of the Invention
It is an object of the present invention to provide an apparatus and method that overcomes many of the foregoing problems. One set of advantages of the present invention is that the wavelength readout signal is monotonically related to the wavelength of the incident beam over ranges that can be in excess of 40 nm, is stable to better than xc2x17 pm over this range, and with minor improvements could reach  less than xc2x13 pm without altering the optical path length, and the speed at which wavelengths are read is only limited to the speed of the photodetection system.
Other objects of the present invention are:
a) to provide an inexpensive apparatus and method for measuring the wavelength or the polarization of light,
(b) to provide apparatus for measuring the wavelength or the polarization of light that can be readily miniaturized,
(c) to measure the wavelength or the polarization of light without knowing the approximate wavelength or the polarization in advance,
(d) to provide a feedback signal that can be used to lock the wavelength output or the polarization state of an optical light source such as a single-mode laser or optical parametric oscillator,
(e) to provide an inexpensive means for measuring the wavelength or the polarization of optical signals from spectroscopic sources used in environmental and chemical analysis,
(f) to provide a means for measuring the wavelength or the polarization fluctuations of a light source, and
(g) to provide a means for measuring the wavelength irrespective of the polarization of the laser light that is incident on the unit,
(h) to provide a means for measuring the polarization of a light field irrespective of the wavelength of the laser light that is incident on the unit.
The objectives and advantages of the invention are achieved by an optical measurement system that directs the laser light into two predetermined paths that travel through a beamsplitter configuration that is optimized for wavelength monitoring or polarization monitoring. The light in each of the paths is detected by a photodetector that provides photocurrent that is received by log amplifiers which provide the logarithm of the photocurrent intensity. A subtractor receives the log outputs of the two photocurrents and provides a signal indicative of wavelength or polarization of the laser, depending on the beamsplitter characteristics.
More particularly, the apparatus includes an optical system comprising an optical beamsplitter that divides the incident laser beam into two beams; one spectral filter that acts to reflect the longer laser wavelengths more strongly than the shorter wavelengths, or a mirror that reflects all of the light of the first beam; one spectral filter that acts to reflect the shorter laser wavelengths more strongly than the longer wavelengths, or a mirror that reflects all the light of the second beam; photodetectors to capture the light from the two beam paths and generate photocurrents; log-amplifiers for receiving said photocurrents and providing the logarithm of the two photocurrents; and a subtractor for subtracting the logarithm of the two photocurrents and provide an output signal indicative of wavelength.
The analog electronic system may be comprised of one photodetector preamplifier for each of the photodetectors; one logarithmic amplifier for each of the photodetected signals; and a difference amplifier for subtracting the two logarithmic signals. A digital signal processing system can be used to replace the analog electronics, if the functionality is maintained.